See those little dots climbing the crystals that look like ants? Those are people.

Casual spelunkers can't enter the cave without risking their lives,
unless they have the right breathing mask and cooling suit. Stifling
temperatures of 122 degrees F combined with almost 100 percent humidity
posed a deadly challenge for early explorers. Such conditions mean that
every breath can lead to fluid condensing inside the lungs, which
represents a less-than-ideal scenario for continued breathing.

But the cave's wonders prove alluring once you get past the dangers. The BBC's
new series "How the Earth Made Us" accompanies Iain Stewart, a
geologist at the University of Plymouth in the UK, on a trip into the
Naica silver mine.

Take a good look at the footage below, because the Cave of Crystals
is destined to disappear once the mine owners decide to end operations
and pull out their pumps. Then the mines and cave alike will flood once
more.